In the art of manufacturing semiconductor products through photolithographic processes, the demand for smaller and smaller minimum circuit feature sizes is increasing. Reducing the circuit feature sizes requires more accurate optical systems which, in turn, requires a more stable mount for the system optics having a high resonance frequency.
To date mirrors have been mounted in cells or housings using well known tangent bar techniques. That is, three bars mounted at one end tangent to the outer diameter of the mirror, and mounted at the other end to the mirror housing. Such a technique does provide an effective way to mount a mirror but has drawbacks. Namely, after the mirror is mounted it may be desirable to move the mirror relative to the housing. Such is the case where the mirror must be adjusted relative to the optical system of which it is a part. Such adjustments inevitably impart stresses from the tangent bars to the mirror which distort the surface figure of the mirror. The prior art mounting techniques provide no precedent for mounting mirrors to housings which are at once flexible, stable and have a high resonance frequency.
A further requisite to attaining the high degree of accuracy required in the future generation of photolithography equipment is the ability to adjust the optical characteristics of the system in real time. This requires being able to move one or more optics along its axis during operation of the equipment, without disturbing the relative optical alignment.
While a number of different types of mirror mounts have been employed heretofore with some success, our contribution to the art is a new system which allows a mirror to be moved relative to its mount, is impervious to temperature excursions, has a high resonance frequency and which allows the mirror focus to be adjusted in real time.